Moisture resistant photovoltaic devices with improved adhesion of barrier film

ABSTRACT

The present invention provides strategies for improving the adhesion between a barrier region, a transparent conductive region, and/or an electrically conductive grid through the use of an adhesion promoting region. The adhesion promoting region is optically transmissive and comprises a metal layer, a metal nitride layer, a metal carbide layer, or a combination thereof and preferably comprises at least one of Cr, Ti, Ta, and Zr or a combination thereof. These strategies are particularly useful in the fabrication of heterojunction photovoltaic devices such as chalcogenide-based solar cells. Adhesion is improved to such a degree that grid materials and dielectric barrier materials can cooperate to provide a hermetic seal over devices to protect against damage induced by environmental conditions, including damage due to water intrusion. The adhesion promoting region also serves as a barrier to the migration of Na, Li, and the lanthanoid series of elements.

PRIORITY

The present nonprovisional patent Application claims priority under 35U.S.C. §119(e) from U.S. Provisional patent application having Ser. No.61/302,687 filed on Feb. 9, 2010, by DeGroot et al. and titled MOISTURERESISTANT PHOTOVOLTAIC DEVICES WITH IMPROVED ADHESION OF BARRIER FILM,wherein the entirety of said provisional patent application isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to photovoltaic devices of the typeincorporating a conductive collection grid that facilitates ease ofmaking external electrical connections, and more particularly tochalcogenide-containing photovoltaic devices with improved adhesionbetween a barrier layer and an adjacent layer in such devices, whereinthe improved adhesion helps provide the devices with enhanced moistureresistance.

BACKGROUND OF THE INVENTION

Both n-type chalcogenide compositions and/or p-type chalcogenidecompositions have been incorporated into components of heterojunctionphotovoltaic devices. The p-type chalcogenide compositions have beenused as the photovoltaic absorber region in these devices. Illustrativep-type, photovoltaically active chalcogenide compositions often includesulfides and/or selenides of at least one or more of aluminum (Al),copper (Cu), indium (In), and/or gallium (Ga). More typically at leasttwo or even all three of Cu, In, and Ga are present. Such materials arereferred to as CIS, CIAS, CISS, CIGS, and/or CIGSS compositions, or thelike (collectively CIGS compositions hereinafter).

Absorbers based upon CIGS compositions offer several advantages. As one,these compositions have a very high cross-section for absorbing incidentlight. This means that CIGS-based absorber layers that are very thin cancapture a very high percentage of incident light. For example, in manydevices, CIGS-based absorber layers have a thickness in the range offrom about 2 μm to about 3 μm. These thin layers allow devicesincorporating these layers to be flexible. This is in contrast tosilicon-based absorbers. Silicon-based absorbers have a lowercross-section for light capture and generally must be much thicker tocapture the same amount of incident light. Silicon-based absorbers tendto be rigid, not flexible.

The n-type chalcogenide compositions, particularly those incorporatingat least cadmium, have been used in photovoltaic devices as bufferlayers. These materials generally have a band gap that is useful to helpform a p-n junction proximal to the interface between the n-type andp-type materials. Like p-type materials, n-type chalcogenide layers canbe thin enough to be used in flexible photovoltaic devices.

These chalcogenide based photovoltaic cells frequently also includeother layers such as transparent conductive layers and window layers.

Heterojunction photovoltaic cells, especially those based on p-type andn-type chalcogenides, are water sensitive and can unduly degrade in thepresence of too much water. Also, the thinner, flexible layers arevulnerable to thermal and other delamination or cracking stresses.Delamination and cracking not only can undermine device performance, butthe resultant delamination and cracking also can exacerbate moistureintrusion. Therefore, to enhance service life, strong adhesion betweendevice components is important to resist delamination, cracking, andmoisture intrusion.

To protect heterojunction photovoltaic solar cells, especiallychalcogenide-based solar cells, from detrimental moisture degradation,one or more hermetic barrier films can be deposited over the devices.FIG. 1 schematically illustrates one such approach. This devicecomprises a support 2, a backside electrical contact 3, an absorberlayer 4, a buffer layer 5, a transparent conductive layer 6, anelectrically conductive grid 7 and a barrier layer 8.

The barrier layer 8 of such a device may tend to show poor adhesion tothe top surface(s) of the device. In particular, the adhesion betweenbarrier materials and underlying transparent conductive materials and/orconductive collection grids may not be as strong as desired.Additionally, the adhesion between the grids and other materials, suchas the TCO compositions, also may be poor. These issues can result inundue delamination 100 or in a rupture of the continuous hermeticbarrier film and/or open pathways 110 allowing water intrusion to reachthe chalcogenide compositions too easily. This can lead to subsequentdevice performance degradation and ultimately failure.

It is known to use silicon nitride films for passivation in the contextof silicon-based solar cells. However, silicon-based solar cells tend tobe thicker and much more rigid than chalcogenide-based cells.Accordingly, interlayer adhesion is much less of an issue in the contextof silicon-based solar cells. Additionally, silicon-based solar cellshave good moisture resistance so that moisture intrusion is much less ofa concern for silicon-based solar cells.

SUMMARY OF THE INVENTION

The present invention provides strategies for improving the adhesionbetween a barrier layer and other layer(s) of a photovoltaic device,such as a transparent conductive layer or region and/or an electricallyconductive grid material. As a consequence, these strategies areparticularly useful in the fabrication of heterojunction solar cellssuch as chalcogenide-based solar cells. Resultant cells are moreresistant to delamination, rupture, and/or moisture intrusion. Devicesprotected by the strategies of the present invention have enhancedservice life. Surprisingly the inventors have found that incorporationof a unique adhesion promoting layer or region between (a) the top layerof the cell (e.g. the transparent conductive layer which has on aportion of its surface the electrical connector (typically anelectrically conducting grid) and (b) a barrier layer provides aphotovoltaic device that has improved protection and most notably forchalcogenide based cells improved moisture resistance as measured by theretained efficiency of the device over time. Adhesion is improved tosuch a degree that the adhesion promoting layer and the dielectricbarrier materials can cooperate to provide a hermetic seal over devicesto protect against damage induced by environmental conditions, includingdamage due to water intrusion. Additionally, the adhesion promotinglayer functions as a barrier to the migration of various elements suchas Na, Li, and the lanthanoid series of elements (Ln) into the barrierlayer.

In one aspect, the invention provides a photovoltaic device thatcomprises:

a) a substrate having a light incident surface and a backside surfaceand comprising at least one photovoltaic absorber, and, on a portion ofthe light incident surface, at least one electrical contact electricallypreferably connected to the absorber;

b) an optically transmissive adhesion promoting region comprising ametal layer or a metal nitride layer or a metal carbide layer, or acombination thereof, over at least a portion of the light incidentsurface of the substrate and the electrical contact; and

c) a dielectric barrier region positioned over the adhesion promotingregion.

In yet another aspect, the invention provides a method of making aphotovoltaic device. The method comprises the steps of:

a) providing a substrate having a light incident surface and a backsidesurface and comprising at least one photovoltaic absorber, and, on aportion of the light incident surface, at least one electrical contactelectrically preferably connected to the absorber;

b) causing an optically transmissive adhesion promoting regioncomprising a metal layer or a metal nitride layer or a metal carbidelayer, or a combination thereof to be formed over at least a portion ofthe light incident surface and electrical contact; and

c) causing a dielectric barrier region to be formed over the adhesionpromoting region.

BRIEF DESCRIPTION OF THE DRAWINGS

The above mentioned and other advantages of the present invention, andthe manner of attaining them, will become more apparent and theinvention itself will be better understood by reference to the followingdescription of the embodiments of the invention taken in conjunctionwith the accompanying drawings, wherein:

FIG. 1 is a schematic cross section of a photovoltaic device of theprior art.

FIG. 2 is a schematic cross section of a photovoltaic device accordingto the present invention.

FIG. 3 is a graph showing relative optical density of model substratesaccording to the invention after exposure to damp heat conditions of115° C./100% relative humidity (RH).

FIG. 4 is a graph showing relative optical density of samples accordingto the invention compared with the relative optical density of a priorart sample after various lengths of exposure to damp heat conditions of115° C./100% relative humidity (RH).

FIG. 5 is an optical microscopic image of a prior art sample after 47hours of exposure to damp heat conditions of 115° C./100% relativehumidity (RH).

FIG. 6 is an optical microscopic image of a sample according to theinvention after 210 hours of exposure to damp heat conditions of 115°C./100% relative humidity (RH).

FIG. 7 is a graph showing the relative optical density of samplesaccording to the invention after 1000 hours of exposure to heat and dampheat conditions of 115° C./100% relative humidity (RH).

FIG. 8 is a plot showing a summary of solar cell survival probabilityversus damp heat (85° C./85% RH) exposure time for cells coated withSi₃N₄/TaN_(x) and with other silicon nitride based barrier layers.

FIG. 9 shows elemental maps (EDX) for N (top, left), O (top, right) andNa (bottom) in a grid region following damp heat exposure for 380 h at115° C./100% RH for comparative samples with silicon nitride barrierlayer (150 nm).

FIG. 10 shows elemental maps (EDX) for N (left) and Na (right) in a gridregion following damp heat exposure for 380 h at 115° C./100% RH forsamples according to the invention, with a tantalum nitride adhesionlayer (10 nm) and a silicon nitride barrier layer (140 nm).

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS

The embodiments of the present invention described below are notintended to be exhaustive or to limit the invention to the precise formsdisclosed in the following detailed description. Rather the embodimentsare chosen and described so that others skilled in the art mayappreciate and understand the principles and practices of the presentinvention. All patents, pending patent applications, published patentapplications, and technical articles cited herein are incorporatedherein by reference in their respective entireties for all purposes.

The photovoltaic cell may be any cell known in the industry having anabsorber and an electrical contact on a portion of the light incidentside. However, preferably the cell is a chalcogenide based cell.

FIG. 2 schematically shows one embodiment of a photovoltaic device 10 ofthe present invention. Device 10 desirably is flexible to allow it to bemounted to surfaces incorporating some curvature. In preferredembodiments, device 10 is sufficiently flexible to be wrapped around amandrel having a diameter of 50 cm, preferably about 40 cm, morepreferably about 25 cm without cracking at a temperature of 25° C.Device 10 includes a light incident face 12 that receives light rays 16and a backside face 14.

Device 10 includes a photovoltaic absorber region 18. In preferredembodiments, region 18 is preferably a chalcogenide-containing absorberregion. Region 18 can be a single integral layer as illustrated or canbe formed from one or more layers. The region 18 absorbs light energyembodied in the light rays 16 and then photovoltaically converts thislight energy into electric energy.

The chalcogenide absorber region 18 preferably incorporates at least oneIB-IIIB-chalcogenide, such as IB-IIIB-selenides, IB-IIIB-sulfides, andIB-IIIB-selenides-sulfides that include at least one of copper, indium,and/or gallium. In many embodiments, these materials are present inpolycrystalline form. Advantageously, these materials exhibit excellentcross-sections for light absorption that allow region 18 to be very thinand flexible. In illustrative embodiments, a typical absorber region 18may have a thickness in the range from about 1 μm to about 5 μm,preferably about 2 μm to about 3 μm.

Representative examples of such IB-IIIB-chalcogenides incorporate one ormore of copper, indium, and/or gallium in addition to selenium and/orsulfur. Some embodiments include sulfides or selenides of copper andindium. Additional embodiments include selenides or sulfides of copper,indium, and gallium. Specific examples include but are not limited tocopper indium selenides, copper indium gallium selenides, copper galliumselenides, copper indium sulfides, copper indium gallium sulfides,copper gallium selenides, copper indium sulfide selenides, coppergallium sulfide selenides, copper indium aluminum selenides and copperindium gallium sulfide selenides (all of which are referred to herein asCIGS) materials. In representative embodiments, CIGS materials withphotovoltaic properties may be represented by the formulaCuIn_((1-x))Ga_(x)Se_((2-y))S_(y) where x is 0 to 1 and y is 0 to 2. Thecopper indium selenides and copper indium gallium selenides arepreferred. The chalcogenide absorber may be doped with other materialssuch as sodium as is known in the art.

The absorber region 18 may be formed by any suitable method using avariety of one or more techniques such as evaporation, sputtering,electrodeposition, spraying, and sintering. One preferred method isco-evaporation of the constituent elements from one or more suitabletargets, where the individual constituent elements are thermallyevaporated on a hot surface coincidentally at the same time,sequentially, or a combination of these to form a precursor to region18. After deposition, the deposited materials may be subjected to one ormore further treatments to finalize the region 18. In many embodiments,CIGS materials have p-type characteristics.

In addition to absorber region 18, device 10 may also include one ormore other components including support 19, backside electrical contactregion 20, buffer region 22, and transparent conducting (TC) region 24,which is preferably a transparent conductive oxide but alternatively maybe a very thin transparent metal film. As shown, each of these regionscan be a single integral layer as illustrated or can be formed from oneor more layers. The support 19 may be rigid or flexible, but desirablyis flexible in those embodiments in which the device 10 may be used incombination with non-flat surfaces. Support 19 may be formed from a widerange of materials. These include glass, quartz, other ceramicmaterials, polymers, metals, metal alloys, intermetallic compositions,paper, woven or non-woven fabrics, combinations of these, and the like.Stainless steel is preferred.

The backside electrical contact region 20 provides a convenient way toelectrically couple device 10 to external circuitry. Contact region 20may be formed from a wide range of electrically conductive materials,including one or more of Cu, Mo, Ag, Al, Cr, Ni, Ti, Ta, Nb, Wcombinations of these, and the like. Conductive compositionsincorporating Mo may be used in an illustrative embodiment. The backsideelectrical contact region 20 also helps to isolate the absorber region18 from the support to minimize migration of support constituents intothe absorber layer. For instance, backside electrical contact region 20can help to block the migration of Fe and Ni constituents of a stainlesssteel support into the absorber region 20. The backside electricalcontact region 20 also can protect the support such as by protectingagainst Se if Se is used in the formation of absorber region 18.

Optional layers (not shown), in addition to the support 19, may be usedproximal to backside face 14 in accordance with conventional practicesnow known or hereafter developed to help enhance adhesion betweenbackside electrical contact region 20 and the support 19 and/or betweenbackside electrical contact region 20 and the absorber region 18.Additionally, one or more barrier layers (not shown) also may beprovided over the backside electrical contact region 20 to help isolatedevice 10 from the ambient and/or to electrically isolate device 10. Oneor more additional layers (not shown) may be deposited onto the backsideof the support for a variety of reasons, including helping preventselenization of the substrate during fabrication of the cell. Such oneor more layers typically include molybdenum.

The device 10 when based upon chalcogenide materials often is providedwith a heterojunction structure in contrast to silicon-basedsemiconductor cells that have a homojunction structure. A heterojunctionmay be formed between the absorber region 18 and the TC region 24buffered by buffer region 22. An optional window region (not shown) alsomay be present. Each of these regions is shown as a single integrallayer, but can be a single integral layer as illustrated or can beformed from one or more layers.

Buffer region 22 generally comprises an n-type semiconductor materialwith a suitable band gap to help form a p-n junction proximal to theinterface between the absorber region 18 and the buffer region 22.Suitable band gaps for the buffer region 22 generally are in the rangefrom about 1.7 eV to about 3.6 eV when the absorber layer is a CIGSmaterial having a band gap in the range from about 1.0 to about 1.6 eV.CdS has a band gap of about 2.4 eV.

Illustrative buffer layer embodiments generally may have a thickness inthe range from about 10 nm to about 200 nm.

A wide range of n-type semiconductor materials may be used to formbuffer region 22. Illustrative materials include selenides, sulfides,and/or oxides of one or more of cadmium, zinc, lead, indium, tin,combinations of these and the like, optionally doped with materialsincluding one or more of fluorine, sodium, combinations of these and thelike. In some illustrative embodiments, buffer region 22 is a selenideand/or sulfide including cadmium and optionally at least one other metalsuch as zinc. Other illustrative embodiments would include sulfidesand/or selenides of zinc. Additional illustrative embodiments mayincorporate oxides of tin doped with material(s) such as fluorine. Awide range of methods, such as for example, chemical bath deposition,partial electrolyte treatment, evaporation, sputtering, or otherdeposition technique, can be used to form buffer region 22.

As noted, device 10 may include an optional window region or layer. Awindow region can help to protect against shunts and also may protectbuffer region 22 during subsequent deposition of the TC region 24. Thewindow region may be formed from a wide range of materials and often isformed from a resistive, transparent oxide such as an oxide of Zn, In,Cd, Sn, combinations of these and the like. An exemplary window materialis intrinsic ZnO. A typical window region may have a thickness in therange from about 10 nm to about 200 nm, preferably about 50 nm to about150 nm, more preferably about 80 nm to about 120 nm.

The TC region 24 is interposed between the buffer region 22 and lightincident surface 12 and is electrically coupled to the buffer region 22to provide a top conductive electrode for the device 10. In manysuitable embodiments where the TC region is a transparent conductiveoxide (TCO), the TCO layer has a thickness in the range from about 10 nmto about 1500 nm, preferably about 150 nm to about 200 nm. As shown, theTCO region 24 is in direct contact with the buffer region 22. As anexample of another option, a window region may be interposed between TCOregion 24 and buffer region 22. One or more intervening layersoptionally may be interposed for a variety of reasons such as to promoteadhesion, enhance electrical performance, or the like.

A wide variety of transparent conducting oxides (TCO) or combinations ofthese may be incorporated into the transparent conductive region 24.Examples include fluorine-doped tin oxide, tin oxide, indium oxide,indium tin oxide (ITO), aluminum doped zinc oxide (AZO), zinc oxide,combinations of these, and the like. In one illustrative embodiment, thetransparent conductive region 24 is indium tin oxide. TCO layers areconveniently formed via sputtering or other suitable depositiontechnique.

The transparent conductive region 24 may alternatively be a very thinmetal film (e.g., a metal film having a thickness greater than about 5nm and more preferably greater than about 30 nm. Additionally, thetransparent conductive region is preferably less than about 200 nmthick, more preferably less than about 100 nm thick. Theserepresentative embodiments result in films that are sufficientlytransparent to allow incident light to reach the absorber region 20).Preferably, the transparent conductive layer is a transparent conductiveoxide. As used herein, the term “metal” refers not only to metals, butalso to metal admixtures such as alloys, intermetallic compositions,combinations of these, and the like. These metal compositions optionallymay be doped. Examples of metals that could be used to form thin,optically transparent layers 30 include the metals suitable for use inthe backside electrical contact 28, combinations of these, and the like.

Adhesion promoting region 26, dielectric barrier region 28, andelectrically conductive collection grid 30 are positioned over thesubstrate TCO region 24. The grid desirably at least includes conductivemetals such as nickel, copper, silver, and the like and/or combinationsthereof. In one illustrative embodiment, the grid has a dual layerconstruction comprising nickel and silver. Since these materials are nottransparent, they are deposited as a grid of spaced apart lines so thatthe grid occupies a relatively small footprint on the surface (e.g., insome embodiments, the grid occupies about 5% or less, even about 2% orless, or even about 1% or less of the total surface area associated withlight capture to allow the photoactive materials to be exposed toincident light). The adhesion promoting layer 26 and the barrier region28 are each shown as a single layer. However, these regions can beformed from more than one layer if desired.

As an overview of the methods of the present invention for forming theseconstituents of device 10, at least a portion of the adhesion promotingregion 26 is formed prior to formation of at least a portion of thebarrier region 28. Preferably, at least substantially the entireadhesion promoting region 26 is formed before the barrier region 28 isformed. Additionally, adhesion promoting layer 26 can block themigration of elements from layers or regions below the barrier regionthat would negatively impact the performance of the barrier layer. Suchelements include, for example, Na, Li, and the lanthanoid (Ln) series ofelements.

Advantageously, the methodologies of the present invention enhance theadhesion quality of the interface between the barrier region 28 and atleast one of the grid 30 and the TCO region 24 in the context ofphotovoltaic devices and in particular flexible chalcogenide-based suchas CIGS-based devices. This methodology provides enhanced protectionagainst delamination and moisture intrusion into the device. Deviceperformance and life are extended as a result. In preferred embodiments,the device of the invention retains at least 90% of its initialefficiency after exposure to 85° C. and 85% relative humidity for a timeof at least 1000 hours.

The adhesion promoting region or layer 26 is formed from one or moremetals, metal nitrides and/or metal carbides, wherein the adhesionpromoting layer preferably contains at least one of Cr, Ti, Ta, and Zror a combination thereof. Furthermore, the adhesion promoting region 26is suitably thin to be optically transmissive. By this is meant region26 has an optical transmittance of greater than or equal to about 70%between 400 nm and 1300 nm. Preferably, adhesion promoting layer has anoptical transmittance of greater that or equal to 80% in the same range.

The adhesion promoting layer is preferably deposited via magnetronsputtering. Where a preferred TaN_(x) barrier layer is to be formed, thebarrier layer preferably is deposited via reactive magnetron sputteringusing a Ta target and a mixture of nitrogen (N₂) and argon gas. The molefraction of N₂ in the gas feed is preferably more than 0.1, morepreferably more than 0.2 and preferably more than 0.6. Prior to thedeposition, a suitable base pressure in the chamber is in the range fromabout 1×10⁻⁸ to about 1×10⁻⁵ Torr. The operating pressure at whichsputtering occurs desirably is in the range from about 2 mTorr to about10 mTorr.

Adhesion promoting region 26 may have a wide range of thicknesses,provided that if region 26 is too thick, then transparency may undulysuffer without providing sufficient extra performance. Illustrativeembodiments of adhesion promoting region 26 may have a thickness lessthan about 200 nm, preferably less than about 100 nm, more preferablystill less than about 50 nm and most preferably less than about 20 nm.Additionally, illustrative embodiments of the adhesion promoting region26 can have a thickness of more than about 1 nm, preferably more thanabout 2 nm.

Dielectric barrier region 28 is formed from one or more suitabledielectric materials that have sufficiently low dielectric constants tohelp electrically isolate TCO region 24 from the ambient environmentexcept in those locations where electric contact is desired through thegrid 30 to TCO 24 at, for example, interface 32. In many embodiments,dielectric barrier region 28 has a dielectric constant in the range of 2to about 120, preferably 2 to about 50, more preferably 3 to about 10.Additionally, dielectric region 28 also desirably provides barrierprotection against water vapor intrusion. In many embodiments,dielectric barrier region 28 is characterized by a water vaportransmission rate (WVTR) in the range of about 10⁰ to about 10⁻⁵g/m²·day, but is most preferably less than about 5×10⁻⁴ g/m²·day. TheWVTR for a material may be determined according to the methodologydescribed in ASTM E 96 or in other tests such as the calcium test (Wolfet al. Plasma Processes and Polymers, 2007, 4, S185-S189).

The dielectric barrier region 28 may be formed from a variety ofmaterial(s). Preferably, the materials used in barrier region 28 arenonporous. The barrier coatings useful in this invention preferablyexhibit optical transmittance ≧70% in the transmission wavelength range400-1300 nm and more preferably exhibit ≧85% transmission in the samerange.

Dielectric barrier region 28 may have a wide range of thicknesses. Iftoo thin, then the electric insulating properties and protection againstmoisture intrusion may not be as robust as might be desired. If toothick, then transparency may unduly suffer without providing sufficientextra performance. Furthermore, if too thick, the dielectric barrierregion 28 may be more susceptible to cracking. Balancing these concerns,illustrative embodiments of barrier region 28 may have a thickness ofless than about 2000 nm, preferably less than about 1000 nm, morepreferably less than about 500 nm, more preferably still less than about250 nm, and most preferably less than about 150 nm. Additionally,illustrative embodiments of the dielectric barrier region 28 can have athickness of more than about 10 nm, and more preferably more than about50 nm.

Dielectric barrier region 28 can be selected from a group of metaloxides, carbides, nitrides and the like or combinations thereof. In onepreferred embodiment, the barrier material is an oxide and/or nitride ofsilicon. These embodiments provide excellent dielectric and moistureprotection. In some embodiments, dielectric barrier region 28 preferablyis formed from silicon nitride or a material incorporating silicon,nitrogen, and oxygen (a silicon oxy nitride). In other embodiments inwhich dielectric barrier region 28 is formed from two or more sublayers,a first sublayer may be formed from silicon nitride, and a secondsublayer may be formed from a silicon oxy nitride. When two or moresublayers are used, it is preferred that the bottom layer (i.e., thelayer in contact with the TCO layer) be silicon nitride.

Representative embodiments of silicon nitride may be represented by theformula SiN_(x), and representative embodiments of silicon oxy nitridemay be represented by the formula SiO_(y)N_(z). In these formulae, x ispreferably greater than about 1.2, more preferably greater that about1.3, and preferably less than about 1.5, more preferably less than about1.4; y is preferably greater than 0, and preferably less than about 0.8,even more preferably less than about 0.5 and still more preferably lessthan about 0.05; and z is greater than about 0.8, preferably greaterthan about 1, and less than about 1.4, more preferably less than about1.3. Other illustrative ranges for x, y, and z are those wherein x is inthe range from about 1.2 to about 1.5, preferably about 1.3 to about1.4; y is preferably in the range from greater than 0 to about 0.8,preferably from about 0.1 to about 0.5; and z is in the range from about0.8 to about 1.4, preferably about 1.0 to about 1.3. Desirably, x, y,and z are selected so that the barrier region 34, or each sublayerthereof as appropriate, has a refractive index in the range from about1.80 to about 3. As an example of one suitable embodiment, siliconnitride of the formula SiN_(1.3) and having a refractive index of 2.03would be suitable in the practice of the present invention.

Adhesion promoting region 26 and dielectric barrier region 28 can beformed on the device 10 in a variety of ways. For example, they may bedeposited via reactive magnetron sputtering using techniques known inthe art.

As an option, the adhesion promoting region 26 and the dielectricbarrier region 28 also may be prepared by other methodologies, includingbut not limited to low temperature vacuum methods known to those skilledin the art including chemical vapor deposition (CVD), plasma-enhancedchemical vapor deposition (PECVD), atomic layer deposition (ALD) andothers.

Grid 30 can be formed from a wide range of electrically conductingmaterials, but most desirably are formed from one or more metals, metalalloys, or intermetallic compositions. Exemplary contact materialsinclude one or more of Ag, Al, Cu, Cr, Ni, Ti, combinations of these,and the like. A grid 30 incorporating Ag or Ag/Ni contacts is preferred.

An optional region (not shown) may include one or more additionalbarrier layers provided over the dielectric barrier region 28 to helpfurther protect device 10. In many modes of practice, these additionalbarrier layers, if any, are incorporated into device 10 after desiredelectrical connections are made to grid 30.

The present invention will now be described with reference to thefollowing illustrative examples. In the first of these examples themethod of accelerated aging was performed on representative test couponscomprising a thin optically transmissive bottom layer of aluminum. Theeffective WVTR was measured by monitoring changes in optical densityover time after exposure to heat and humidity. The accelerated exposurewas conducted in an All-American 25× steam sterilizer equipped with anexcess pressure relief valve. NANOPURE™ water was used exclusively inthe pressure vessel to avoid contamination. For each sample, the initialoptical density was measured at 9 points equally distributed across thesurface of the sample. The samples were then placed vertically in asample holder and introduced into the pressure vessel for testing. Thetemperature was set to 115° C. using an external temperature controller.The temperature reading did not exceed ±1° C. of the set point. At thistemperature, the pressure inside of the vessel was approximately 12psig. The samples were exposed for the desired duration and then removedfrom the pressure vessel and the optical density measured at the samepositions as initially measured. The samples were then returned to thepressure vessel and the process repeated. Testing was discontinued ifthe samples failed, that is when the normalized optical densitydecreased by more than 10% of the initial optical density value. Opticaldensity measurements were carried out using an X-RITE™ 361T transmissiondensitometer using a 3 mm aperture. Density measurements were taken forboth orthogonal and ultraviolet responses, although only the former wereused for comparison of relative moisture barrier performance. Furtherillustrative examples are provided in which the photovoltaic performanceof devices was monitored over time during exposure to damp heatconditions of 85° C./85% RH. CMS-based devices, prepared with andwithout the adhesion promoting layer 26 made substantially as set forthin Example 5, are subjected to damp-heat, 85° C./85% RH, environmentalweathering conditions as specified in IEC standard 61646. During theexposure, the cells are positioned vertically on a stainless steelfixture situated above a pool of DI water within a lab oven held at 85°C.±5° C. Prior to collecting the I-V characteristic measurement, thesamples rest in a dry nitrogen purged box for at least 12 hrs. Then,just before collecting the I-V characteristic measurement, the samplesare light soaked for at least 5 minutes using a SpectraNova solarsimulator. Immediately following this measurement the devices arereturned to the damp-heat environment for the next test period. Thisprocess is repeated for each time period.

The device efficiency is extracted from a current-voltage (I-V)characteristic curve that is measured before and after each step using aClass AAA solar simulator. The I-V characteristic measurement apparatusand procedure meet the requirements specified in the IEC 60904 (parts1-10) and 60891 standards. For each I-V measurement, electrical contactis established using a 5-μm-radius tungsten probe tip placed in contactwith the collection grid bus bar and the Molybdenum coated back side wasgrounded thru an Au coated brass platen. During the I-V characteristicmeasurement, the temperature of the platen and the device is maintainedat 25° C. Prior to the measurement the Xe arc lamp is given 15 minutesto stabilize. Then, the lamp irradiance is set to AM1.5 1000 W/m2 usinga calibrated silicon reference device with BK-7 filter. The uncertaintyin the efficiency measurement is ±4% relative.

Example 1

Test coupons according to the invention were prepared. A thin film ofaluminum (about 30 nm thick) was sputter-deposited onto three 1″×1″pieces of soda-lime glass. This was followed by a layer of indium tinoxide (ITO) (about 130 nm thick). Indium tin oxide (ITO) films wereprepared using a custom RF magnetron sputter chamber from a 100 mmdiameter, 5 mm thick ITO ceramic target (90 wt % In₂O₃, 10 wt % SnO₂)using gas flows of argon (14 sccm) and oxygen (2 sccm), controlled usingmass flow controllers, to achieve a working gas pressure of 2.8 mTorr.The substrate temperature was held at 150° C. during deposition. A maskwas applied to shield an area of the sample and expose only the area tobe covered by a conductive grid. Layers of Ni and then Ag having a totalthickness of about 1600 nm were sequentially deposited by E-beamevaporation. Prior to evaporation, the chamber base pressure is reducedto <2×10⁻⁶ Torr. All depositions can be carried out at 9.0 kV, whilecurrent values are 0.130 and 0.042 Amps for Ni and Ag, respectively. Thedeposition rates can be controlled in process using a Maxtek 260 quartzcrystal deposition controller at 2.0 Å/s and 15.0 Å/s for Ni and Ag,respectively. Ni shots (99.9999%, obtained from International AdvancedMaterials) can be evaporated from a 7 cc graphite crucible, while Agpellets (99.9999%, Alfa Aesar) can be evaporated from a 7 cc molybdenumcrucible. An ultra-thin layer (about 10 nm thick) of tantalum nitrideadhesion promoting layer was sputter deposited over the silver grid andthe ITO layer. The TaN_(x) layer was prepared using a custom RFmagnetron sputter chamber from a 50 mm diameter, 6.4 mm thick Ta targetusing N₂ sputtering gas. TaN_(x) films were deposited over athree-minute period at a power of 140 watts and a pressure of 4 mTorr.The base pressure prior to deposition is less than 1.0×10⁻⁵ Torr. Thedeposition of TaN_(x) was followed by a layer of silicon nitride (Si₃N₄)of about 150 nm thickness, prepared by reactive sputtering from a 50 mmSi target using an Ar:N₂ (50:50) sputtering gas mixture at 4 mTorr.

The samples were then placed in a pressure cooker at 115° C. and 100%relative humidity for accelerated exposure testing. Changes in theoptical density of the Al film were monitored periodically as describedabove and are reported in FIG. 3. The optical density of the Al film wasmeasured to determine the extent of oxidation of the Al to aluminumoxide due to exposure to moisture. The formation of a more transparentaluminum oxide layer leads to a decrease in optical density. As seen inFIG. 3, after over 536 hours at 115° C./100% RH, the optical density ofall three samples remained at about 100%.

Example 2

Samples according to the invention were prepared as described in Example1 except that different adhesion promoting materials were used incombination with the dielectric barrier of Si₃N₄ as the top surface ofthe device. The Si₃N₄ barrier layer was about 150 nm thick.Additionally, a comparative device was prepared that did not employ anadhesion promoting layer.

The following materials were used as the adhesion promoting layer:

Thickness Example Adhesion Promoting Mat'l. (ca. nm) 2A Cr 5 2B Ti 10 2CTa 10 Comparative None —

The samples were placed in the pressure cooker as described above andoptical density was periodically measured. The results are shown in FIG.4. The comparative example shows a marked decrease in optical densityover time while Examples 2A, 2B, 2C showed no measurable decrease inoptical density.

Example 3

Comparative samples were prepared that comprised a 1″×1″ soda limeglass, an Al film (about 30 nm thick), a silver grid (about 1000 nmthick) and a film of silicon nitride (about 150 nm thick) usingconditions described in Example 1 for the respective layers. The sampleswere placed in the pressure cooker at 115° C. and 100% relative humidityfor accelerated exposure testing. After 47 hours of exposure, thesamples were removed and top down images using optical microscopy wereobtained. Similarly, samples of Example 1 were imaged after 210 hours ofexposure in the pressure cooker. FIG. 5 clearly shows the damage to thesilicon nitride barrier and the grid on the Comparative Samples. FIG. 6clearly shows that comparative regions of the sample comprising atantalum nitride adhesion layer between the top surface of the device(TCO/grid) and the dielectric silicon nitride barrier layer areessentially undamaged.

Example 4

Sample coupons according to the invention were prepared as described inExample 1. A thin layer (10 nm) of tantalum nitride followed by adielectric barrier of Si₃N₄ was applied as the top surface of thesamples (about 140 nm thick). The samples were placed in the pressurecooker at 115° C. and 100% relative humidity for accelerated exposuretesting. FIG. 7 shows that there was no measurable decrease in opticaldensity after 1000 hours of exposure.

Example 5

Photovoltaic devices according to the invention were prepared on 2″square soda-lime glass substrates, 0.7 mm thick. A layer of molybdenumwas sputter deposited at 200 W, 6×10³ mbar on the glass substrate, to afinal thickness of about 750-800 nm. CIGS absorber layer was depositedby a multi-stage metal co-evaporation process based on a three stageprocess practiced by NREL (Repins, 2008). A cadmium sulfide buffer layerwas deposited by chemical bath deposition (CBD) by dipping samples intoa mixture of 33 mL 0.015 M CdSO_(4(aq)) and 42 mL 14.5 M NH₄OH_((aq))(concentrated NH₃) at 70° C. After 1 min. 33 mL of 0.75 mL thiourea wasadded and the reaction was allowed to proceed for 7 min. Samples weredried at 110° C. for 30 min., then heated to 200° C. The window layer,i-ZnO, was prepared by RF magnetron sputtering of a ZnO target at 60 Wand 10 mtorr sputtering pressure (0.15% O₂ in Ar sputtering gas) to afinal thickness of about 70 nm. The devices were completed by depositionof ITO, grids, tantalum nitride adhesion layer and silicon nitrideaccording to the procedures described in Example 1. Prior to depositionof the tantalum nitride and silicon nitride layers, individual cellswere isolated by scribing down to, but not through the Mo layer.Comparative examples were prepared without the tantalum nitride adhesionlayer.

FIG. 8 shows a summary of the results of solar cell survival probabilityversus damp heat (85° C./85% RH) exposure time for cells coated withSi₃N₄/TaN, and with other silicon nitride based barrier layers. Theperformance of devices with a TaN_(x) adhesion layer is clearly superiorto those devices lacking this layer. Devices with a TaN_(x) adhesionlayer have a >70% probability of surviving 1000 h under accelerated dampheat conditions. Several devices survived greater than 3000 h under thesame conditions.

Example 6

Sample coupons on soda lime glass with Al/ITO and Ni/Ag grids wereprepared as described in Example 1. A thin layer (10 nm) of tantalumnitride followed by a dielectric barrier of silicon nitride (140 nm) wasapplied to one set of sample coupons according to the conditionsdescribed in Example 1, while only Si₃N₄ (150 nm) was applied to asecond set for comparison. The samples were placed in the pressurecooker at 115° C. and 100% relative humidity for accelerated exposuretesting for 380 h. FIG. 9 shows that the comparative samples (withsilicon nitride barrier only) showed significant oxidation of thesilicon nitride layer (evidenced by the reduction in N content in thegrid region and associated increase in oxygen content). FIG. 9 alsoshows the presence of sodium at the surface of the device with a similardistribution as oxygen. In contrast, FIG. 10 shows that samplesaccording to the invention (with the inclusion of the thin tantalumnitride layer) showed no measurable sodium at the surface of the devicein the grid region.

What is claimed is:
 1. A photovoltaic device having a light incidentface and a backside surface, wherein the photovoltaic device comprises:a) a support; b) at least one photovoltaic absorber positioned over thesupport to receive light via the light incident face of the device,wherein the photovoltaic absorber comprises a chalcogenide-containingabsorber having a thickness in the range from 1 micrometer to 5micrometers; c) a transparent conductive layer interposed between the atleast one photovoltaic absorber and the light incident face of thedevice; d) at least one electrical contact positioned over and inelectrical communication with the transparent conductive layer and theat least one photovoltaic absorber, wherein the at least one electricalcontact comprises a layer of conductive material that is less extensivethan the underlying transparent conductive layer; e) an opticallytransmissive adhesion promoting region comprising Cr, Ti, Ta, TiN_(x),TaN_(x), TiC_(x), TaC_(x) or is a multilayer structure comprised of acombination thereof over at least a portion of the electrical contactand the transparent conductive layer, wherein the optically transmissiveadhesion promoting region has a thickness less than 50 nanometers; andf) an optically transmissive dielectric barrier region positioned overthe adhesion promoting region, wherein the optically transmissivedielectric barrier region comprises silicon nitride, silicon oxide,silicon carbide, silicon oxynitride, and/or combinations thereof andwherein the optically transmissive adhesion promoting region is incontact with at least one electrical contact, the transparent conductivelayer, and the optically transmissive dielectric barrier region.
 2. Thephotovoltaic device of claim 1, wherein the absorber is achalcogenide-containing absorber comprising copper, indium, andoptionally gallium.
 3. The photovoltaic device of claim 1, wherein thephotovoltaic article retains at least 90% of its initial efficiencyafter exposure to 85° C. and 85% relative humidity for a time of atleast 1000 hours.
 4. The photovoltaic device of claim 1, wherein theelectrical contact comprises an electrical grid.
 5. The photovoltaicdevice of claim 4, wherein the electrical grid comprises a discontinuouslayer of conductive material.
 6. The photovoltaic device of claim 1,wherein the electrical contact comprises Ag or Ag/Ni.
 7. Thephotovoltaic device of claim 1, wherein the device further comprises abuffer layer between the transparent conductive layer and the absorber.8. The photovoltaic device claim 1, wherein the adhesion promotingregion has an optical transmittance of greater than or equal to 70%between 400 nm and 1300 nm.
 9. The photovoltaic device of claim 1,wherein the optical transmittance of the barrier region is greater thanor equal to 70% between 400 nm and 1300 nm.
 10. The photovoltaic deviceof claim 1, wherein the adhesion promoting region further functions as abarrier to the migration of Na, Li, and lanthanoid series elements intothe dielectric barrier region.
 11. The photovoltaic device of claim 1,wherein the at least one electrical contact comprises a layer ofnon-transparent conductive material.
 12. The photovoltaic device ofclaim 1, wherein the optically transmissive dielectric barrier regionhas a water vapor transmission rate value in the range of 10⁻⁵g/(m²*day) to 10⁰ g/(m²*day).